Photolithographic processes are essential to the field of microelectronics. Accurate, reproducible lines with widths of only a few microns (.mu.m) must be routinely produced on integrated circuit boards or panels by exposing photosensitive materials to ultraviolet light through masks which have been photolithographically fabricated.
One type of photosensitive material used to produce photographic images in integrated circuit panels is a photoresist. Photoresists are photosensitive polymers which are resistant to attack by chemicals and vapors, and are thus suitable for the photoengraving techniques which form the basis of microelectronics. Photoresists further exhibit film-forming properties and are reasonably adherent to various surfaces.
Photoresists are classified as either positive photoresists or negative photoresists. Negative photoresists become insoluble when exposed to light. Positive photoresists, on the other hand, become soluble when exposed to light. Positive photoresists are generally more expensive to produce and more expensive to use than are negative photoresists. The energy required to expose them is greater, resulting in a longer exposure time and a resulting reduction in throughput. A distinct advantage of positive photoresists is that patterns can be delineated as small as the thickness of the photoresist layer, thereby making it possible to define lines on a panel as small as 1 .mu.m with a 1 .mu.m layer of positive photoresist.
Negative photoresists are available in the form of a dry film sandwiched between a polyester film and a polyolefin film. The photoresist is applied by removing the polyolefin film and using heat and pressure to cause it to adhere to the panel as a laminate.
Processes and systems for laminating photoresists onto a panel for the manufacture of circuit boards are known. One such process involves the application of a liquid to the surfaces of the panel to interact with the photoresist, prior to application of the photoresist. The fluid improves the flow characteristics of the photoresist and improves its adherence to the surface of the panel. This type of wet lamination process is disclosed in U.S. Pat. No. 5,112,428 to Correa, et al.
Such known wet lamination processes, however, suffer from several drawbacks. Typically, the processes involve complex controls to meter liquid flow and complicated roller designs to apply the liquid to the panel. The complex controls necessarily require significant operator intervention to insure that the proper amount of liquid is applied to the panel. Frequent fluctuations in wetting performance are common and result in less than optimum product yields.
In addition, known systems which apply fluid to a panel, to promote adhesion of a material such as a photoresist thereto, include rollers made from, or covered with, materials that may disintegrate during operation. Such disintegration may cause product contamination or mechanical breakdown. Known roller materials which suffer from this drawback include cloth which may separate into fibers. Periodic replacement of these types of rollers result in reduced product yields.
Moreover, such designs do not work well with ordinary tap water, which is an inexpensive, yet effective adhesion promoter for photoresist materials. The high mineral content of ordinary tap water may cause corrosion of the rollers, as well as sedimentary buildup on the rollers and other portions of the device. The use of ordinary tap water with these devices requires separate filtration systems for the water and frequent system preventative maintenance and cleaning to remove mineral deposits.
Still further, known systems do not provide sufficient means for insuring that both the top and the bottom of the panel are wetted adequately. Typical designs are horizontally oriented such that visual verification of the wetting of the bottom of a board passing therethrough is not possible.
It is an object of the present invention, therefore, to provide an apparatus and method for improved wet application of a dry film to a generally planar panel having upper and lower surfaces, which apparatus and method do not require complex controls or excessive operator intervention, and which provide an optimal product yield.
It is a further object of the present invention to provide such an apparatus and method which incorporate roller materials which resist disintegration, which utilize ordinary tap water as a process material, and which facilitate easy visual verification of adequate top and bottom surface wetting.
It is still a further object of the present invention to provide an apparatus and method for uniformly applying a liquid to a generally planar panel having upper and lower surfaces, in applications outside of those relating to printed circuit board manufacture.